The invention relates to a communication system having a connecting line and stations connected thereto for transmitting and receiving information, each station comprising a processor, a transmitting stage and a receiving stage, the processor in each station having free access to the connecting line via the transmitting stage, and each station including a detector for switching off the transmitting station upon detection of the presence of another station requesting and having, respectively, substantially simultaneous access to the connecting line, and to a station suitable therefor.
Such a communication system is described in the magazine "Electronics" dated June 5th, 1980, pages 89 and 90. The advantage of the use of free access is the absence of a central system controller which determines which station is authorized to transmit, as in the event of breakdown in the system controller, which is per se very expensive, the entire communication system may be put out of operation. When there is free access to the system, each station comprises a detector for detecting a station which had previously started transmitting or a station which starts transmitting more or less simultaneously. The above article describes that a station which requests access and detects a different transmitting station waits a predetermined period of time, which time period becomes longer on repeated trials. It is not described how the presence of the other transmitting station is detected. A station which had previously started transmitting may, for example, be detected by means of a carrier detector connected to the connecting line. The detection of another station which starts transmitting more or less simultaneously, that is to say in the same clock pulse period, creates more problems, as now both stations influence the information present on the connecting line, to which information combination in the region of the station, the information the station transmits itself will contribute most.